dt-bindings: pinctrl: Add X1500 bindings.

Add the pinctrl bindings for the X1500 Soc from Ingenic.

Signed-off-by: Zhou Yanjie <zhouyanjie@zoho.com>
Link: https://lore.kernel.org/r/1563076436-5338-6-git-send-email-zhouyanjie@zoho.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>